1. Field of the Invention
The present invention relates to a ballast, or power supply circuit for gas discharge lamps. More particularly, the invention relates to control circuitry which controls the bus voltage in response to varying load conditions without the need for direct sensing of the load conditions.
2. Description of the Related Art
Electronic ballast circuits are known in the art. FIG. 1 illustrates a conventional electronic ballast 10 including an AC source 12, an electromagnetic interference (EMI) filter 14, a pre-regulator stage 16, a DC bus capacitor 18, a lamp driver stage 20 and a load 22. The pre-regulator stage 16 serves two primary functions. First, the pre-regulator stage 16 performs input acurrent shaping (i.e., power factor correction). Second, the pre-regulator stage 16 performs bus voltage regulation. The pre-regulator stage 16 can be implemented in a number of different ways. The most common implementations utilize either a boost converter or a flyback converter. One method of controlling a boost converter to perform pre-regulation is by constructing the boost converter to operate in a critical discontinuous conduction mode, as illustrated in the circuit of FIG. 2a and the waveforms of FIG. 2b. 
Referring now to FIG. 2a, at the beginning of a switching cycle, the inductor current in L1 is zero. At the instant in time transistor Q1 is turned on, the current in inductor L1 ramps up linearly. After a certain time, Q1 is switched off and the current in L1 forces diode D1 to conduct to charge bus capacitor C1. During the time D1 is conducting, the current in L1 ramps linearly downward. At the point at which the current in L1 reaches zero, D1 turns off and the switching cycle repeats.
Power factor correction is achieved in the circuit of FIG. 2a by either turning Q1 on for a constant time, or by switching Q1 off when the current in L1 reaches a value proportional to the input voltage (i.e., a rectified sinusoid). As such, the average value of the current in L1 will be a rectified sine wave in phase with the input voltage, as shown in FIG. 2b. 
The bus voltage, Vbus, is regulated to a constant value by varying the on time of Q1 in order to balance the power delivered by the input and the power delivered to the output. By decreasing the on time, less power is drawn from the input and by increasing the on time, more power is drawn from the input.
The objective of most circuit applications is to control the on time of Q1 such that the bus voltage, Vbus, is maintained at a constant value for all load conditions.
For circuits designed to drive compact high intensity discharge (HID) lamps with a low frequency square wave current, Vbus, may be typically regulated to 400V during steady state operation. FIG. 3 illustrates such a circuit. However, during ignition, a higher bus voltage (i.e., on the order of 500V, (See point A) is required for proper ignition of the lamp (e.g., 250V, See point B). Simply setting the bus voltage, Vbus, to 500V is a sub-optimal solution in that a high bus voltage is only required during ignition. Maintaining a high bus voltage throughout normal steady state operation of a lamp creates stress on the components and increases losses in the circuit. Another solution is to provide circuit means to detect when the lamp is not ignited and increase the bus voltage to 500V for ignition and to reduce the bus voltage gradually to 400V after the lamp is ignited in order to reduce switching losses and component voltage stresses. Typically this requires a direct sensing of the load conditions. That is, the load voltage, load current, or even both the current and voltage must be sensed in order to determine the state of the lamp. This requires additional sensing circuitry and the corresponding controls in order to control the bus voltage to 500V prior to ignition and to 400V after some load condition has been satisfied.
It is also possible to control the bus voltage without sensing load conditions by attempting to guess the proper timing of the ignition sequence. For example, the circuit upon startup can be configured to have Vbus at 500V. After a predetermined time, the bus voltage can be reduced to 400V. However this approach is not preferred in that the ignition behavior of the lamp can never be known or guaranteed.
Therefore a need exists to detect a load condition and adjust the bus voltage accordingly by indirect means without the need for additional sensing circuitry, so that proper ignition behavior is achieved and switching losses and component voltage stresses are reduced.
It is an object of the present invention to provide a high power factor electronic ballast for a gas discharge lamp which can provide bus voltage control in response to varying load conditions without the need for direct sensing of the load conditions.
An electronic ballast for providing load dependent bus voltage regulation according to the present invention which accomplishes the above object includes: an input stage; an output stage; a first controller referred to herein as a slow loop controller configured to regulate the bus voltage to a constant value during steady state operation; a second controller referred to herein as a hysteresis band over voltage protection (OVP) controller for maintaining the bus voltage in a predetermined hysteresis voltage band during open circuit and pre-ignition conditions; and an on-time limiting controller for guaranteeing a minimum on-time for the input stage. The electronic ballast further includes at least one DC bus capacitor for storing energy created from energy imbalances between the input and output stage.
With the above construction, it is possible to regulate the bus voltage to a constant value under steady state conditions using the slow loop controller, and to prevent the bus voltage from increasing in an uncontrolled manner under open circuit and pre-ignition conditions using the over voltage protection controller. This is advantageously achieved without having to directly sense the load condition.